Separation membrane comprising coating layer, method of preparing same, and battery using same

ABSTRACT

Disclosed herein is a high thermal resistant polyolefin-based separator including a coating layer containing polyamic acid.Specifically, the separator includes a polyolefin-based substrate film, and a coating layer containing polyamic acid formed on one or both surfaces of the polyolefin-based substrate film, wherein the polyamic acid contains one or more functional groups selected from the group consisting of a sulfone group, a trifluoromethyl group, an alkyl group, and a phenyl ether group.Also, disclosed herein is an electrochemical battery having improved thermal stability by using the separator including a coating layer containing polyamic acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application based on pending application Ser. No.14/418,785, filed Jan. 30, 2015, which is the U.S. National Phase ofPCT/KR2013/006894 filed Jul. 31, 2013, the entire contents of which arehereby incorporated by reference.

Korean priority Patent Application No. 10-2012-0084536, filed on Aug. 1,2012, and Korean priority Patent Application No. 10-2012-0126384, filedon Nov. 9, 2012, in the Korean Intellectual Property Office, areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a separator including a coating layer,and an electrochemical battery using the same.

BACKGROUND ART

A separator for an electrochemical battery refers to an intermediatemembrane segregating a cathode and an anode from each other in abattery, while continuously maintaining ion conductivity, therebyallowing battery charging and discharging.

Recently, along with weight lightening and miniaturization trend of anelectrochemical battery for high portability of an electronic device, abattery is also required to have high power and large capacity in orderto be used for an electric car, and the like. Thus, a separator for abattery is required to be thin and have light weight, and at the sametime, to have excellent thermal shape stability for production of a highpower battery.

Particularly, in case where a polyolefin-based film is used as asubstrate film of the separator, the film may melt down even atrelatively low temperature, and thus, in order to compensate for such aproblem, a study to improve the thermal resistance of the substrate filmhas proceeded. It is suggested in Korean Patent Registration No.10-0775310, etc. that a coating layer of a mixture of organic andinorganic materials is formed on one or both surfaces of a substratefilm of a separator, in order to improve the thermal resistance of asubstrate film.

Meanwhile, an attempt has been made to improve thermal stability of acoating layer, by using an organic binder having excellent thermalresistance such as polyimide as a coating agent component of aseparator. However, the organic binder having high thermal resistancedoes not dissolve in a low boiling point solvent, which later causes aproblem of not properly performing a drying process of the solvent aftercoating a separator. Moreover, this not only decreases air permeabilityof a separator, but also reduces compatibility with other coating agentcomponents added together so as to make it difficult to be substantiallyutilized as a coating agent.

Therefore, the development of a separator for a battery having excellentthermal resistance and air permeability, by coating the separator withan organic binder having high thermal resistance and also highsolubility in a low boiling point solvent as a coating agent component,is needed.

DISCLOSURE OF INVENTION Technical Problem

An object of the present invention is to provide a separator havingexcellent thermal resistance and drying processability, by utilizingpolyamic acid having excellent solubility in a low boiling point solventas a coating agent component of a separator.

Another object of the present invention is to provide a separator havinga less solvent residual amount in a coating layer so as to haveexcellent air permeability, and improved thermal resistance.

Another object of the present invention is to provide an electrochemicalbattery having excellent thermal stability, by using the separator.

Another object of the present invention is to provide a separatormaintaining an excellent shutdown function of a polyolefin-basedseparator, and having improved thermal resistance and air permeability.

Technical Solution

In one general aspect, a polyolefin-based separator includes a coatinglayer containing polyamic acid.

Specifically, the separator includes a polyolefin-based substrate film,and a coating layer containing polyamic acid formed on one or bothsurfaces of the polyolefin-based substrate film, wherein the polyamicacid has the structure of following Chemical Formula 1 or 2.

wherein

R₁, R₅ and R₇ are, independently of one another, an unsubstituted orsubstituted aromatic hydrocarbon having 6 to 30 carbon atoms; anunsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbonatoms; or an unsubstituted or substituted alicyclic hydrocarbon having 3to 24 carbon atoms. R₁, R₅ and R₇ may be identical to or different fromone another.

In another general aspect, an electrochemical battery includes theseparator, a cathode, an anode, and electrolyte.

In another general aspect, a lithium secondary battery includes theseparator.

Advantageous Effects

The separator of the present invention maintains a shutdown property asit is, while having improved melt-down temperature and excellent thermalresistance.

Further, the separator of the present invention has a less solventresidual amount in a coating layer of the dried separator, thereby notreducing air permeability, and compensates for the thermal sensitivityof polyolefin, thereby enhancing the thermal resistance of theseparator.

The coating composition for a separator of the present invention mayhave excellent solubility in a low boiling point solvent, and mitigate adrying condition, so that simplification of a process and cost savingare possible.

Further, the separator of the present invention has strong resistance tothermal shrinkage which occurs upon overheating of a battery, and thus,in case of utilizing the separator in a battery, the battery hasimproved stability and extended life.

Further, the polyamic acid used in the coating composition of thepresent invention may maintain the dispersibility of inorganic particlesin an inorganic dispersion, so as to improve the preparationprocessability of a coating layer of a mixture of organic and inorganicmaterials.

The effects according to the present invention are not limited by theillustration above, and various effects are included herein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing capacity change with the number of cycle of abattery using the separator according to an exemplary embodiment of thepresent invention.

FIG. 2 is a graph showing the measurement of shutdown temperature of theseparator according to Example 1 (Graph B) and Comparative Example 1(Graph A).

FIG. 3 is a graph showing the result of thermal mechanical analysis(TMA) measurement of the separator according to Example 1 (Graph B) andComparative Example 1 (Graph A).

FIG. 4 is a graph showing the measurement of a high-rate dischargeproperty (C-rate) of a separator according to Example 1 and ComparativeExample 1.

BEST MODE

Hereinafter, the present invention will be described in detail. Sincethe part not disclosed herein may be sufficiently recognized andinferred by a person skilled in the art to which the invention pertainsor in the similar art, a description thereof will be omitted.

The separator according to an exemplary embodiment of the presentinvention includes a coating layer containing high thermal resistantpolyamic acid. Specifically, the separator according to an exemplaryembodiment of the present invention includes a polyolefin-basedsubstrate film, and a coating layer containing polyamic acid formed onone or both surfaces of the polyolefin-based substrate film, wherein thepolyamic acid has the structure of following Chemical Formula 1 or 2.

wherein

R₁, R₅ and R₇ are, independently of one another, an unsubstituted orsubstituted aromatic hydrocarbon having 6 to 30 carbon atoms; anunsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbonatoms; or an unsubstituted or substituted alicyclic hydrocarbon having 3to 24 carbon atoms. R₁, R₅ and R₇ may be identical to or different fromone another.

Specifically, the aromatic hydrocarbon having 6 to 30 carbon atoms maybe represented by following Chemical Formula 3:

wherein Ar₁ to Ar₄ are, independently of one another, unsubstituted orsubstituted arylene having 6 to 15 carbon atoms, and Ar₁ to Ar₄ may beidentical to or different from one another; and X₁ to X₃ are,independently of one another, a single bond, O, S, C(═O), S(═O)₂,C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbonatoms, specifically alkylene having 1 to 4 carbon atoms, orunsubstituted or substituted silylene. For example, the substitutedalkylene or silylene may be mono- or di-substituted by F, OH, CH₃, CF₃and the like.

m, l and o are, independently of one another, 0 or 1. Specifically, if mis 1, and l and o are 0, X₂ and X₃ are a single bond, and Ar₂ isunsubstituted or substituted trivalent arylene having 6 to 15 carbonatoms, and if m and l are 1, and o is 0, X₃ is a single bond, and Ar₃ isunsubstituted or substituted trivalent arylene having 6 to 15 carbonatoms. If m, l and o are all 0, X₁ to X₃ are a single bond, and Ar₁ isunsubstituted or substituted tetravalent arylene having 6 to 15 carbonatoms, for example, unsubstituted or substituted tetravalent phenyleneor naphthylene.

In the Chemical Formula 1 or 2, R₂, R₆ and R₈ are, independently of oneanother, an unsubstituted or substituted aromatic hydrocarbon having 6to 30 carbon atoms; an unsubstituted or substituted aliphatichydrocarbon having 2 to 20 carbon atoms; an unsubstituted or substitutedalicyclic hydrocarbon having 3 to 24 carbon atoms; or —R₃—Ar₅—R₄—(wherein R₃ and R₄ are, independently of each other, alkylene having 1to 5 carbon atoms; and Ar₅ is unsubstituted or substituted arylenehaving 6 to 15 carbon atoms, and if Ar₅ is substituted, it is mono- totri-substituted by CH₃, OH, SH or N₂). R₂, R₆ and R₈ may be identical toor different from one another.

Specifically, in R₂, R₆ and R₈, the unsubstituted or substitutedaromatic hydrocarbon having 6 to 30 carbon atoms may be represented byfollowing Chemical Formula 4:—(Ar₆)—X₄—(Ar₇)_(p)—X₅—(Ar₈)_(q)—X₆—(Ar₉)_(r)—  [Chemical Formula 4]

wherein Ar₆ to Ar₉ are, independently of one another, arylene having 6to 15 carbon atoms, and Ar₆ to Ar₉ may be identical to or different fromone another; and X₄ to X₆ are, independently of one another, a singlebond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substitutedalkylene having 1 to 10 carbon atoms, specifically alkylene having 1 to4 carbon atoms, or unsubstituted or substituted silylene. For example,the substituted alkylene or silylene may be mono- or di-substituted byF, OH, CH₃, CF₃ and the like.

p, q and r are, independently of one another, 0 or 1. If p is 1, and qand r are 0, X₅ and X₆ are a single bond, and Ar₆ and Ar₇ are,independently of each other, unsubstituted or substituted phenylene ornaphthylene. If p and q are 1, and r is 0, X₆ is a single bond. If p, qand r are all 0, X₄ to X₆ are a single bond.

In the Chemical Formula 4, X₄ may be in an ortho-, meta-, orpara-position to an amine group (—NH—) of the Chemical Formula 1 or 2.Specifically, it may be in a meta-position, and in case of being in ameta-position to an amine group, its solubility may be improved.

In the Chemical Formula 1, n is an integer of 30 to 10000, an integer of100 to 1000, an integer of 50 to 500, or an integer of 50 to 300.

In the Chemical Formula 2, x is an integer of 15 to 5000, and y is aninteger of 15 to 5000, specifically, x is an integer of 50 to 500, and yis an integer of 50 to 500, and more specifically, x is an integer of 25to 250, and y is an integer of 25 to 250.

According to an exemplary embodiment of the present invention, in theChemical Formula 1 or 2, R₁, R₂, R₅, R₆, R₇ and R₈ may be, independentlyof one another, an unsubstituted or substituted aromatic hydrocarbonhaving 6 to 18 carbon atoms; an unsubstituted or substituted alicyclichydrocarbon having 6 to 24 carbon atoms; or an unsubstituted orsubstituted aliphatic hydrocarbon having 2 to 17 carbon atoms, and inthe aromatic hydrocarbon of the Chemical Formula 3 or 4, Ar₁ to Ar₄, orAr₆ to Ar₉ may be, independently of one another, an unsubstituted orsubstituted arylene having 4 to 10 carbon atoms, specifically phenyleneor naphthylene Herein, in the Chemical Formula 3, m, l and o may be all0, or m may be 1, and l and o may be 0, and in the Chemical Formula 4,p, q and r may be all 1, or q may be 1, and q and r may be 0

The polyamic acid compound according to another exemplary embodiment ofthe present invention may be the compound wherein in the aromatichydrocarbon of the Chemical Formula 3 or 4, Ar₁ to Ar₄ areunsubstituted, and Ar₆ to Ar₉ are, independently of one another,unsubstituted or substituted. If any one or more of Ar₆ to Ar₉ aresubstituted, they may be substituted by CH₃, CF₃, OH, F or the like, forexample, CH₃ or CF₃.

The polyamic acid compound according to another exemplary embodiment ofthe present invention may be the compound wherein in the ChemicalFormula 1 or 2, R₁, R₅ and R₇ are an unsubstituted or substitutedphenylene or naphthylene, an unsubstituted or substituted alicyclichydrocarbon having 4 to 12 carbon atoms, an aliphatic hydrocarbon having4 to 7 carbon atoms, or two or three unsubstituted or substituted phenylgroups in the form of being connected by a single bond, O, S, C(═O),S(═O)₂, C(═)NH, unsubstituted or substituted alkylene having 1 to 10carbon atoms, unsubstituted or substituted silylene, and the like. Thetwo or three unsubstituted or substituted phenyl groups may be,specifically, independently of each other, in the form of beingconnected by a single bond; C(═O); S(═O)₂; alkylene having 1 to 3 carbonatoms unsubstituted or mono- or di-substituted by CH₃ or CF₃; orsilylene unsubstituted or mono- or di-substituted by CH₃ or CF₃. Morespecifically, the connecting group of the two or three phenyl groups maybe a single bond, C(═O), C(CH₃)₂ or C(CF₃)₂. Alternatively, two phenylgroups may be connected by the connecting group. R₂, R₆ and R₈ may be anunsubstituted or substituted alicyclic hydrocarbon having 4 to 12 carbonatoms, an unsubstituted or substituted aliphatic hydrocarbon having 4 to17 carbon atoms, or two to four unsubstituted or substituted phenylgroups in the form of being connected by a single bond, O, S, C(═O),S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10carbon atoms, unsubstituted or substituted silylene, and the like. Thetwo to four unsubstituted or substituted phenyl groups may be,specifically, independently of each other, in the form of beingconnected by a single bond; S(═O)₂; O; C(═O); alkylene having 1 to 3carbon atoms unsubstituted or mono- or di-substituted by CH₃ or CF₃; orsilylene unsubstituted or mono- or di-substituted by CH₃ or CF₃. Morespecifically, the connecting group of the two to four phenyl groups maybe a single bond, C(═O), S(═O)₂, O, C(CH₃)₂ or C(CF₃)₂. In particular,the connecting group may be C(═O), S(═O)₂ or O. For example, two phenylgroups may be connected by the connecting group.

The polyamic acid compound according to another exemplary embodiment ofthe present invention may be the compound wherein in the ChemicalFormula 1 or 2, R₁ and R₅ are unsubstituted or substituted phenylene ornaphthylene, and R₂, R₆, R₇ and R₈ are two to four unsubstituted orsubstituted phenyl groups in the form of being connected by a singlebond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substitutedalkylene having 1 to 10 carbon atoms, unsubstituted or substitutedsilylene, or the like.

The polyamic acid having the structure of the Chemical Formula 1 or 2may represent adequate solubility in the low boiling point solvent (thesolvent having a boiling point less than 150° C.). Specifically, if anorganic binder component of the separator coating agent has lowsolubility in the low boiling point solvent, the preparation of thecoating agent usable in a general coating method itself may bedifficult, and on the contrary, if it has unduly high solubility in thelow boiling point solvent, battery safety may be rather decreased due tothe risk of dissolving the separator in the electrolyte solution of abattery. The polyamic acid having the structure of the Chemical Formula1 or 2 has adequately controlled solubility in a low boiling pointsolvent, and thus, is suitable as a coating composition of a separator.The polyamic acid having the structure of the Chemical Formula 1 or 2has excellent thermal resistance and solubility in a low boiling pointsolvent, thereby securing the thermal resistance of the separator withpolyamic acid itself without proceeding with imidation separately, andalso dispenses with carrying out imidation at high temperature, therebyavoiding the damage of a polyolefin layer which is a lower substrate,and thus, a coating layer may be formed on a polyolefin-based substrate.Further, since its solubility in a low boiling point solvent isexcellent, a high temperature drying process under a severe conditionfor removing a high boiling point solvent is not necessary, and thus, acoating layer may be formed on a polyolefin substrate. Though polyimidehas excellent thermal resistance, it does not dissolve in a low boilingpoint solvent, so that it has not been substantially utilized as acoating agent component of the separator. Specifically, in a separatorincluding a coating layer, if a large amount of solvent remains in thecoating layer after drying, the separator will have reduced adhesion,and also decreased air permeability, and thus, not function properly asa separator. Therefore, in case where the separator is intended to becoated by a general coating method (in particular, dip coating), a lowboiling point solvent has been used as a coating agent solvent, in orderto facilitate drying of the solvent. However, in case of polyimide,since it does not dissolve in the low boiling point solvent, it has beendifficult to be utilized in a separator coating layer in the prior art,in spite of its excellent thermal resistance. Thus, the problems in theprior art are intended to be solved in the present invention byintroducing polyamic acid which has excellent thermal resistance andeasily dissolves in a low boiling point solvent.

The term used herein, ‘aromatic hydrocarbon having 6 to 30 carbon atoms’includes all the cases where the aromatic hydrocarbon is present alone,two aromatic hydrocarbons are joined to form a condensed ring, or two ormore aromatic rings are not joined to each other, but connected byanother connecting group. The aromatic hydrocarbon present alone isexemplified by a divalent or tetravalent phenylene group, and twoaromatic hydrocarbons joined to each other to form a condensed ring areexemplified by a divalent or tetravalent naphthylene group.

The term used herein, ‘arylene having 6 to 15 carbon atoms’ includes anaromatic hydrocarbon having 6 to 15 carbon atoms wherein the aromatichydrocarbon is present alone, or two aromatic hydrocarbons are joined toeach other to form a condensed ring, and the arylene is exemplified by aphenylene or naphthylene group. The arylene may be di-, tri- ortetravalent, herein.

The term used herein, ‘alicyclic hydrocarbon having 3 to 24 carbonatoms’ refers to a saturated or partially unsaturated hydrocarbon groupcontaining 1 to 3 rings having 3 to 8 carbon atoms, respectively. Forexample, as the alicyclic hydrocarbon having 6 to 20 carbon atoms, acyclohexyl, cycloheptyl, cyclohexenyl, or 1,2,3,4-tetrahydronaphthalenegroup may be mentioned.

The term used herein, ‘aliphatic hydrocarbon having 2 to 20 carbonatoms’ refers to a saturated or partially unsaturated, straight-chainedor branched-chained, di- or tetravalent hydrocarbon group. Specifically,it may refer to a saturated, straight-chained or branched-chained, di-or tetravalent hydrocarbon group. As an example of the aliphatichydrocarbon having 2 to 20 carbon atoms, ethyl, butyl, pentyl, hexyl,1,1-dimethylbutyl or the like may be used.

In case where the term, ‘unsubstituted or substituted’ is used herein,unless otherwise stated, a group may be unsubstituted or substitutedseveral times by F, OH, SH, CH₃, CF₃, NH₂ or the like. For example, agroup may be mono- to tri-substituted by F, OH, SH, CH₃, CF₃, NH₂ or thelike.

In case where R₂, R₆ or R₈ is phenylene, or R₂, R₆ or R₈ contains aphenylene group as a part herein, it may be, independently of oneanother, connected in an ortho-, meta- and para-position.

Specifically, in the Chemical Formulae 1 and 2, R₁, R₅ and R₇ may beselected from the group consisting of following Chemical Formulae A1 toA43.

Specifically, in the Chemical Formulae 1 and 2, R₂, R₆ and R₈ may beselected from the group consisting of following Chemical Formulae B1 toB74.

The polyamic acid compound according to another exemplary embodiment ofthe present invention may include one or more functional groups selectedfrom the group consisting of a sulfone group, a trifluoromethyl group,an alkyl group and a phenyl ether group. For example, one or moresulfone, trifluoromethyl, alkyl and/or phenyl ether groups may beincluded in the molecule, and both sulfone and trifluoromethyl groupsmay be included. If the polyamic acid includes one or more sulfone,trifluoromethyl, alkyl and/or phenyl ether groups, it may have moreincreased solubility in a low boiling point solvent. Specifically, thepolyamic acid represented by any one of following Chemical Formulae 5 to7 may be used.

The polyamic acid having the structure of the Chemical Formula 5 hasexcellent solubility in a low boiling point solvent, and specifically itmay represent excellent solubility in a solvent composition containingacetone being a low boiling point solvent and N,N-dimethyl acetamide(DMAc) being a high boiling point solvent in a weight ratio of about9.5:0.5 or less, that is, in a weight ratio of acetone to DMAc of9.5:0.5 or less. More specifically it may represent excellent solubilityin a solvent composition containing acetone and DMAc in a weight ratioof acetone to DMAc of 9:1 or less, or 8.75:1.25 or less.

The polyamic acid having the structure of the Chemical Formula 6 hasexcellent solubility in a low boiling point solvent, and specifically itmay represent excellent solubility in a solvent composition containingacetone being a low boiling point solvent and DMAc being a high boilingpoint solvent in a weight ratio of about 8:2 or less, that is, in aweight ratio of acetone to DMAc of 8:2 or less. More specifically, itmay represent excellent solubility in a solvent composition containingacetone and DMAc in a weight ratio of acetone to DMAc of 7.5:2.5 orless.

The polyamic acid having the structure of the Chemical Formula 7 hasexcellent solubility in a low boiling point solvent, and specifically itmay represent excellent solubility in a solvent composition containingacetone being a low boiling point solvent and DMAc being a high boilingpoint solvent in a weight ratio of about 7.5:2.5 or less, that is, in aweight ratio of acetone to DMAc of 7.5:2.5 or less. More specifically,it may represent excellent solubility in a solvent compositioncontaining acetone and DMAc in a weight ratio of acetone to DMAc of 6:4or less.

Further, polyamic acid having the structure wherein an amic acidrepeating unit containing a phenyl ether group (x) (hereinafter,referred to as a ‘unit’) and an amic acid unit containing a sulfonegroup (y) are repeated, or polyamic acid having the structure wherein anamic acid unit containing a sulfone group but not containing atrifluoromethyl group (x) and an amic acid unit containing sulfone andtrifluoromethyl groups (y) are repeated, may be used. Specifically, thepolyamic acid represented by following Chemical Formula 8 or 9 may beused:

wherein more specifically, a ratio x:y is 5:5 to 1:9. Within the rangeof the ratio, the polyamic acid solubility in the low boiling pointsolvent (e.g., acetone) may be increased.

The polyamic acid having the structure of the Chemical Formula 8 mayspecifically, represent excellent solubility in a solvent compositioncontaining acetone being the low boiling point solvent and DMAc beingthe high boiling point solvent in a weight ratio of about 8:2 or less.More specifically, it may represent excellent solubility in a solventcomposition containing acetone and DMAc in a weight ratio of acetone toDMAc of 7.5:2.5 or less.

wherein a ratio x:y is 9:1 to 7:3. The polyamic acid within the range ofthe ratio may have not too high solubility in an electrolyte solutionand the like, and only sufficiently high solubility in the low boilingpoint solvent. The polyamic acid having the structure of the ChemicalFormula 9 has excellent solubility in the low boiling point solvent, andspecifically, may represent excellent solubility in a solventcomposition containing acetone being the low boiling point solvent andDMAc being the high boiling point solvent in a weight ratio of about9.5:0.5 to 5:5.

In the Chemical Formulae 5 to 9, the sulfone group may be a substituentin an ortho-, meta- or para-position to an amine group, for example, ina meta-position. In case where the sulfone group is a substituent in themeta-position, polyamic acid solubility in the low boiling point solventmay be increased.

The polyamic acid according to exemplary embodiments of the presentinvention may have a weight average molecular weight (Mw) of 50,000 to100,000. In case of having the molecular weight range, it may haveincreased solubility in the low boiling point solvent, and improvedthermal resistance.

The polyamic acid of the Chemical Formula 1 may be prepared by a methodknown in the art to react an anhydride containing R₁ with diaminecontaining R₂. The polyamic acid of the Chemical Formula 2 may beprepared by a method known in the art to react an anhydride containingR₅ with diamine containing R₆, and an anhydride containing R₇ withdiamine containing R₈.

The non-limiting example of the anhydride containing R₁, R₅ or R₇ mayinclude pyromellitic dianhydride,4,4′-(hexafluoroisopropylidene)diphthalic dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,4,4′-carbonyldiphthalic dianhydride, 1,2,3,4-butanetetracarboxylicdianhydride, 4,4′-oxydiphthalic dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride,3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride,4,4′-isopropylidenediphthalic dianhydride,1,2,4,5-cyclohexanetetracarboxylic dianhydride,1,2,3,4-cyclopentanetetracarboxylic dianhydride, or the like.

The non-limiting example of the diamine containing R₂, R₆ or R₈ mayinclude 3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenyl sulfone,1,6-hexamethylenediamine, 4,4′-oxydianiline, 4,4′-methylenedianiline,1,3-phenylenediamine, 1,4-phenylenediamine,2,2-bis(3-amino-4-methylphenyl)hexafluoropropane, meta-xylenediamine,para-xylenediamine, 3,3′-(hexafluoroisopropylidene)dianiline,4,4′-(hexafluoroisopropylidene)dianiline),3-[3-(3-aminophenyl)sulfonylphenyl]sulfonylaniline,2,2′-bis(trifluoromethyl)benzidine, 1,16-hexadecanediamine,1,4-cyclohexyldiamine, 3,3′-bis(trifluoromethyl)benzidine,ortho-tolidine, 2,3,5,6-tetramethyl-1,4-phenylenediamine,2,5-dimethyl-1,4-phenylenediamine,4,4′-diamino-3,3′-dimethyldiphenylmethane,2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, or the like

The polyamic acid having the structure of the Chemical Formula 1 or 2may represent adequate solubility in the low boiling point solvent (thesolvent having a boiling point less than 150° C.).

Specifically, if an organic binder component of the separator coatingagent has low solubility in the low boiling point solvent, thepreparation of the coating agent usable in a general coating methoditself may be difficult, and on the contrary, if it has unduly highsolubility in the low boiling point solvent, battery safety may berather decreased due to the risk of dissolving the separator in theelectrolyte solution of a battery.

Thus, in one exemplary embodiment of the present invention, there isprovided a coating composition having adequately controlled solubilityin the low boiling point by using the polyamic acid having the structureof the Chemical Formula 1 or 2.

The polyamic acid used in the present invention may be contained in 1 to30% by weight, more specifically 1 to 20% by weight, and for example, 1to 15% by weight, based on the weight of the coating composition. Withinthe range, the polyamic acid may sufficiently serve as an organic bindercomponent of the coating agent, and sufficiently impart high thermalresistance to the coating composition.

The low boiling point solvent used in the present invention refers to asolvent having a boiling point less than 150° C. The non-limitingexamples of the low boiling point solvent usable in the presentinvention may include acetone, tetrahydrofuran (THF), and the like.These may be used alone or in a mixture of two or more, and for example,acetone may be used. Since acetone has a significantly low boiling pointof about 56.5° C., if it is used as a solvent of a coating agent, thecoating layer may be easily dried, so that the air permeability of theseparator becomes excellent, and also deterioration of the physicalproperties due to the residual solvent may be prevented.

According to one embodiment of the present invention, additionally tothe low boiling point solvent, a high boiling point solvent may be usedtogether as the solvent of the coating composition.

The high boiling point solvent usable in the present invention refers toa solvent having boiling point of 150° C. or more. The non-limitingexample of the high boiling point solvent usable in the presentinvention may include dimethylformamide (DMF), dimethylsulfoxide (DMSO),dimethylacetamide (DMAc), dimethylcarbonate (DMC), N-methylpyrrolidone(NMP), or the like. These may be used alone or in a mixture of two ormore.

As to the contents of the low boiling point solvent and the high boilingpoint solvent, the weight ratio of the low boiling point solvent (X) tothe high boiling point solvent (Y) (X:Y) may be 9.5:0.5 to 5:5,specifically 9.0:1.0 to 5:5, and more specifically 8:2 to 5:5.

In case of adjusting the content ratio of the low boiling point solventand the high boiling point solvent to the above range, the polyamic acidmay be sufficiently dissolved to facilitate the preparation of thecoating agent, and the coating layer formed on the substrate film mayalso be easily dried. That is, the solvent may remain in a small amountin the dried coating layer of the separator (for example, 500 ppm orless), so that the air permeability of the separator will not bedecreased.

The total content of the solvent including the low boiling point solventand the high boiling point solvent may be 20 to 99% by weight,specifically 50 to 95% by weight, and more specifically 70 to 95% byweight, based on the weight of the coating composition. If the solventis contained within the range, the coating agent may be easily prepared,and the drying process of the coating layer may be performed well.

The inorganic particles used in the present invention are notspecifically limited, and any inorganic particles generally used in theart may be used. The non-limiting examples of the inorganic particlesusable in the present invention include Al₂O₃, SiO₂, B₂O₃, Ga₂O₃, TiO₂,SnO₂ or the like. These may be used alone or in a mixture of two ormore. The inorganic particles used in the present invention may be, forexample, Al₂O₃ (alumina).

The size of the inorganic particle used in the present invention is notspecifically limited, and the average particle diameter may be 1 to2,000 nm, or 100 to 1,000 nm. In case of using the inorganic particleswithin the size range, dispersibility and coating processability of theinorganic particles in a coating solution may be prevented from beingreduced, and the thickness of the coating layer may be appropriatelyadjusted to prevent mechanical property deterioration and increase inelectric resistance. Further, the size of the pores produced in theseparator may be appropriately adjusted to lower a probability ofcausing an internal short-circuit upon charging and discharging abattery.

In the preparation of the coating composition, the inorganic particlesmay be used in the form of an inorganic dispersion being dispersed in anappropriate solvent. The appropriate solvent is not specificallylimited, and any solvent generally used in the art may be used. As anappropriate solvent to disperse the inorganic particles, acetone may beused, for example.

The inorganic dispersion may be prepared by a general method without aspecial limitation, and for example, in a manner of adding Al₂O₃ in aproper amount to acetone, and milling it to be dispersed with a beadsmill.

In the preparation of the inorganic dispersion, the content of theinorganic particles may be 10 to 40% by weight, specifically 20 to 30%by weight, based on the weight of the dispersion. If the inorganicparticles are contained within the range, their heat dissipationproperty may be sufficiently exhibited, and the separator coated usingthem may have effectively suppressed thermal shrinkage.

The content of the inorganic dispersion may be 10 to 70% by weight,specifically 20 to 60% by weight, and more specifically 30 to 50% byweight, based on the weight of the coating composition of the presentinvention. Within the range, the inorganic particles may be expected toshow a sufficient heat dissipation property, and the content of anorganic binder is also relatively properly adjusted, and thus, theadhesion of the separator may be secured above a certain level.

According to an exemplary embodiment of the present invention, thecoating composition may further include a binder in addition to thepolyamic acid. The binder may be one selected from the group consistingof polyvinylidene fluoride (PVdF) homopolymer, polyvinylidenefluoride-hexafluoropropylene copolymer (PVdF-HFP),polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone,polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide,cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose and acrylonitrilestyrene-butadiene copolymer, alone or in amixture thereof. Specifically, polyvinylidene fluoride homopolymerand/or polyvinylidene fluoride-hexafluoropropylene copolymer may beincluded.

In case of further including polyvinylidene fluoride homopolymer, theviscosity and adhesion of the coating composition may be improved toassist the inorganic particles to be uniformly dispersed, and also, acoating layer having high adhesion may be formed on a substrate film toincrease the stability of the separator. The weight average molecularweight of the polyvinylidene fluoride-hexafluoropropylene copolymerusable in the present invention may be 1,000,000 g/mol or less, or forexample, 1,000,000 to 1,200,000 g/mol, but not specifically limitedthereto. Within the molecular weight range, the adhesion between thecoating layer and the polyolefin-based substrate film may be enhanced,so that the heat-sensitive polyolefin-based substrate film may beeffectively prevented from shrinking by heat, and additionally, theadhesion between the coating layer and an electrode may also be improvedto prevent the short-circuit of a cathode and an anode. Further, in caseof using the polyvinylidene fluoride homopolymer within the molecularweight range, the polyvinylidene fluoride homopolymer may dissolve welleven with a small amount of DMF, thereby facilitating the drying of thecoating layer.

Further, for example, in case of coating the separator with a coatingagent further containing polyvinylidene fluoride-hexafluoropropylenecopolymer, the electrolyte impregnation property of the separator may beimproved to produce a battery having excellent electrical output. Theweight average molecular weight of the polyvinylidene fluoride copolymerusable in the present invention may be 800,000 g/mol or more, or forexample, 600,000 to 800,000 g/mol, but not specifically limited thereto.In case of using the polyvinylidene fluoride-hexafluoropropylenecopolymer within the molecular weight range, a separator having asufficiently improved electrolyte impregnation property may be prepared,and using the separator, a battery efficiently generating electricaloutput may be produced.

In the polyvinylidene fluoride-hexafluoropropylene copolymer used in thepresent invention, each content of the polyvinylidene fluoride andhexafluoropropylene is not specifically limited, but thehexafluoropropylene may be contained in 0.1 to 40% by weight, based onthe total weight of the copolymer.

The method of preparing a coated separator according to one exemplaryembodiment of the present invention may include applying a coatingcomposition containing polyamic acid, a low boiling point solvent, and ahigh boiling point solvent on one or both surfaces of a polyolefin-basedsubstrate film, and drying it to form a coating layer.

The preparation of a coating composition containing polyamic acid, a lowboiling point solvent, and a high boiling point solvent in the presentinvention may include mixing 1 to 30% by weight of the polyamic acid,based on the total weight of the coating composition, with 70 to 99% byweight of the low boiling point solvent and the high boiling pointsolvent, based on the total solvent, and stirring it at 10 to 40° C. for30 minutes to 5 hours.

The coating composition may further include inorganic particles.Therefore, the preparation of the coating composition may include mixingthe polyamic acid, the low boiling point solvent, the high boiling pointsolvent, and the inorganic particles, and stirring it at 10 to 40° C.for 30 minutes to 5 hours. Herein, the content of the inorganicparticles may be 10 to 40% by weight, based on the total weight of thecoating composition.

Alternatively, the coating composition may be prepared by dispersing theinorganic particles in a dispersion medium to prepare an inorganicdispersion, and mixing it with a high molecular solution containing thepolyamic acid, the low boiling point solvent and the high boiling pointsolvent. In case of separately preparing the inorganic dispersion assuch, the dispersion property and solution stability of the inorganicparticles and the polyamic acid may be improved. Therefore, in anotherembodiment, the coating composition of the present invention may beprepared by mixing the polyamic acid ingredient and the inorganicparticles prepared in the state of being dissolved or dispersed in anappropriate solvent, respectively.

For example, the coating composition may be prepared in a manner ofpreparing solutions of polyamic acid, polyvinylidene fluoridehomopolymer and/or polyvinylidene fluoride-hexafluoropropylene copolymereach dissolved in an appropriate solvent, and an inorganic dispersion inwhich inorganic particles are dispersed, respectively, then mixing themwith an appropriate solvent.

After mixing the polyamic acid solution, the inorganic dispersion, andthe solvent, the mixture may be sufficiently stirred using a ball mill,a beads mill, a screw mixer, or the like to prepare a coatingcomposition in the form of a mixture.

According to another embodiment of the present invention, there isprovided a separator in which the coating composition is coated on oneor both surfaces of the polyolefin-based substrate film.

The method to coat the polyolefin-based substrate film with the coatingagent is not specifically limited, and any method generally used in theart may be used. The non-limiting example of the coating method mayinclude a dip coating, a die coating, a roll coating, a comma coatingmethod, or the like. These methods may be applied alone or in acombination of two or more. The coating layer of the separator of thepresent invention may be formed by, for example, a dip coating method.

The coating layer of a mixture of organic and inorganic materials of thepresent invention may have a thickness of 0.01 to 20 μm, specifically 1to 15 μm. Within the thickness range, the coating layer may be formed tohave an adequate thickness to obtain excellent thermal stability andadhesion, and the separator may be prevented from being unduly thickentirely, thereby suppressing an increase of internal resistance of abattery.

The coating layer may be dried by warm air, hot air or low humidity air,or vacuum dried, or dried by irradiating far-infrared radiation,electron beam or the like in the present invention. In addition, thedrying temperature depends on the kind of the solvent, but may begenerally 60 to 120° C. The drying time also depends on the kind of thesolvent, but may be generally 1 minute to 1 hour. In a specific example,the drying may be carried out at 90 to 120° C. for 1 to 30 minutes, or 1to 10 minutes In the present invention, the solvent may be effectivelyremoved even under a condition of reduced drying time and lower dryingtemperature as the above by using the polyamic acid having excellentsolubility in the low boiling point solvent as a coating compositioncomponent.

After drying, the residual amount of the low boiling point solvent andthe high boiling point solvent in the coated separator may be 500 ppm orless. Specifically, the residual amount of the low boiling point solventand the high boiling point solvent in the coated separator may be 400ppm or less. For example, after drying, the low boiling point solventmay not remain, and the high boiling point solvent may remain in 500 ppmor less in the coating layer.

It is preferred that the substrate film used in the separator of thepresent invention is polyolefin-based. The non-limiting example of thepolyolefin-based substrate film may include a polyethylene substratefilm, a polypropylene substrate film, or the like.

In case of the separator for a secondary battery, a substrate filmhaving a shutdown function is preferred, and the polyolefin-basedsubstrate film used in the separator of the present inventioncorresponds to the substrate film having an excellent shutdown function.

The polyolefin-based substrate film used in the present invention may beselected from the group consisting of for example, a single polyethylenemembrane, a single polypropylene membrane, a double-layeredpolyethylene/polypropylene membrane, a triple-layeredpolypropylene/polyethylene/polypropylene membrane, and a triple-layeredpolyethylene/polypropylene/polyethylene membrane.

The polyolefin-based substrate film may have a thickness of 1 to 40 μm,more specifically 1 to 30 μm, and still specifically 1 to 20 μm. In caseof using the substrate film within the thickness range, the preparedseparator may have an adequate thickness which is enough to prevent ashort-circuit of a cathode and an anode of a battery, but not enough toincrease the internal resistance of a battery.

The polyamic acid of the present invention is not imitated in thecoating layer, and may be present in the form of polyamic acid.

The residual amount of the organic solvent in the dried coating layer ofthe separator of the present invention may be 500 ppm or less. Theorganic solvent residual amount refers to a sum of the residual amountsof the low boiling point solvent and the high boiling point solvent, ifboth solvents are used.

The solvent residual amount in the dried coating layer of 500 ppm orless in the present invention does not numerically include a value lessthan zero, and technically refers to a positive (+) value of 0 to 500ppm.

The dried coating layer of the separator of the present invention refersto a coating layer dried by the drying process at 70 to 120° C.,specifically 100 to 120° C. for 1 to 20 minutes, or 1 to 10 minutes,more specifically 1 to 2 minutes, or dried at 10 to 30° C. for 6 to 48hours, after coating the coating agent on the polyolefin-based substratefilm.

In case where the solvent residual amount in the dried coating layer ofthe separator is 500 ppm or less, the followings may be prevented: aproblem caused by an excessive amount of the solvent remaining in thecoating layer, that is, an organic binder component not representingsufficient adhesion, and a problem of not effectively suppressing thethermal shrinkage of the substrate film due to reduced adhesion of thecoating layer, which accordingly functions as a factor hindering batteryperformance upon charging/discharging a battery which causes theshort-circuit of an electrode when a battery overheats.

The organic solvent remaining in the dried coating layer of the presentinvention may have a boiling point higher than the melting point of thesubstrate film of the present invention.

When measuring a temperature at which a separator breaks by pulling theseparator coated with the polyamic acid with a force of 0.005 N at aheating rate of 5° C./min (see ASTM E 831), the breaking temperature maybe 180° C. or more. Within the range, the separator does not shrink welleven at high temperature, so that the stability of the separator and abattery may be improved. Therefore, in another exemplary embodiment ofthe present invention, there is provided a separator including apolyolefin-based substrate film; and a coating layer containing polyamicacid formed on one or both surfaces of the substrate film, wherein theseparator has a breaking temperature of 180° C. or more under acondition of 0.005 N, and a heating rate of 5° C./min.

The separator coated with the polyamic acid of the present invention mayhave, after being left at 200° C. for 1 hour, thermal shrinkage in amachine direction (MD) or a transverse direction (TD) of 20% or less,specifically 10% or less, more specifically 5% or less, respectively.The thermal shrinkage may be 5% or less, for example. Within the range,the short-circuit of an electrode may be effectively prevented toimprove the stability of a battery.

The separator coated with the polyamic acid of the present invention mayhave, after being left at 150° C. for 1 hour, thermal shrinkage in amachine direction (MD) or a transverse direction (TD) of 15% or less,specifically 13% or less, more specifically 10% or less, respectively.The thermal shrinkage may be 5% or less, for example. Within the range,the short-circuit of an electrode may be effectively prevented toimprove the stability of a battery.

The method to measure the thermal shrinkage of the separator is notspecifically limited, and any method generally used in the art may beused.

The non-limiting example of the method of measuring the thermalshrinkage of the separator is as follows: After a prepared separator iscut into a size of about 5 cm (MD)×about 5 cm (TD), and stored in achamber at 200° C. for 1 hour, the shrinkage in MD and TD directions ofthe separator is measured to calculate the thermal shrinkage.

The thermal shrinkage at 150° C. may be measured in the same manner asthe above method, except that the chamber at 200° C. is replaced withthe chamber at 150° C.

The separator of the present invention may have thermal resistancetemperature of 200° C. or more. If the separator has the thermalresistance temperature of 200° C. or more, an electrode short-circuitphenomenon by heat may be effectively suppressed, thereby manufacturinga battery having high thermal safety. ‘Thermal resistance temperature’used herein, refers to temperature to which when the separator isexposed for 10 minutes, the shrinkage of the separator in MD/TDdirections is less than 5%.

According to another embodiment of the present invention, there isprovided an electrochemical battery including a porous polyolefin-basedseparator containing the coating layer of a mixture of organic andinorganic materials, and a cathode and an anode, and filled withelectrolyte.

The kind of the electrochemical battery is not specifically limited, andmay be any kind known to the art.

The electrochemical battery of the present invention may be,specifically, a lithium secondary battery such as a lithium metalsecondary battery, a lithium ion secondary battery, a lithium polymersecondary batter, a lithium ion polymer secondary battery, or the like.

The method of manufacturing the electrochemical battery of the presentinvention is not specifically limited, and any method generally used inthe art may be used.

The non-limiting example of the method of manufacturing theelectrochemical battery is as follows: The battery may be manufacturedin a manner of disposing the polyolefin-based separator including thecoating layer of a mixture of organic and inorganic materials between acathode and an anode of the battery, and then filling it with anelectrolyte solution.

The electrode forming the electrochemical battery of the presentinvention may be prepared in the form of binding an electrode activematerial to an electrode current collector by a method generally used inthe art.

The cathode active material of the electrode active material used in thepresent invention is not specifically limited, and any cathode activematerial generally used in the art may be used.

The non-limiting example of the cathode active material may includelithium manganese oxide, lithium cobalt oxide, lithium nickel oxide,lithium iron oxide, a lithium complex oxide combining those oxides, orthe like.

The anode active material of the electrode active material used in thepresent invention is not specifically limited, and any anode activematerial generally used in the art may be used.

The non-limiting example of the anode active material may include alithium adsorption material such as a lithium metal or lithium alloy,carbon, petroleum coke, activated carbon, graphite, or other carbons.

The electrode current collector used in the present invention is notspecifically limited, and any electrode current collector generally usedin the art may be used.

The non-limiting example of the materials of the cathode currentcollector of the electrode current collector may include foil preparedfrom aluminum, nickel or a combination thereof, or the like.

The non-limiting example of the materials of the anode current collectorof the electrode current collector may include foil prepared fromcopper, gold, nickel, a copper alloy or a combination thereof, or thelike.

The electrolyte solution used in the present invention is notspecifically limited, and any electrolyte for an electrochemical batterygenerally used in the art may be used.

The electrolyte solution may be one wherein a salt having a structuresuch as A⁺B⁻ is dissolved or dissociated in an organic solvent.

The non-limiting example of A⁺ may include a cation selected from thegroup consisting of an alkali metal cation such as Li⁺, Na⁺ or K⁺, or acombination thereof.

The non-limiting example of B⁻ may include an anion selected from thegroup consisting of an anion such as PF₆ ⁻, BF4⁻, Cl⁻, Br⁻, ClO₄ ⁻, AsF₆⁻, CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻ or C(CF₂SO₂)₃ ⁻, or a combinationthereof.

The non-limiting example of the organic solvent may include propylenecarbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC),dimethyl carbonate (DMC), dipropylcarbonate (DPC), dimethyl sulfoxide,acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran,N-methyl-2-pyrrolidone (NMP), ethylmethylcarbonate (EMC),γ-butyrolactone, or the like. These may be used alone or in a mixture oftwo or more.

Hereinafter, the present invention is described in more detail, bydescribing the following Examples, Comparative Examples, andExperimental Examples. However, the Examples, Comparative Examples, andExperimental Examples are only illustrative of the present invention,and the disclosure of the present invention is not construed as beinglimited thereto.

EXAMPLES 1 TO 20 Preparation of Separator Having a Coating LayerContaining Polyamic Acid Formed Thereon Example 1

(1) Preparation of Coating Composition

0.5 mol of 3,3′-diamino diphenyl sulfone (DDS), andN,N-dimethylacetamide (DMAc) were placed in a four-neck flask equippedwith a stirrer, a temperature controlling device, a nitrogen gasinjection device, and a condenser while passing nitrogen therethrough,and dissolved with stirring. Next, 0.5 mol of pyromellitic dianhydride(PMDA) in a solid state was added to the solution, and the solution wasvigorously stirred. Herein, the solid content by mass ratio was 20% byweight, and the reaction proceeded for 24 hours while maintaining thetemperature less than 25° C. to prepare a polyamic acid solution.

In order to prepare an inorganic dispersion, 25% by weight of Al₂O₃(LS235, Nippon Light Metal Company, Ltd.) was added to acetone (DaejungChemical & Metals Co., Ltd.), and milled at 25° C. for 3 hours using abeads mill to be dispersed, thereby preparing an inorganic dispersion.

The thus prepared, polyamic acid solution and inorganic dispersion weremixed in a weight ratio of polyamic acid solution:inorganicdispersion:N,N-dimethylacetamide (DMAc):acetone=1:4.8:0.8:3.4, andstirred at 25° C. for 2 hours with a power mixer to prepare a coatingcomposition.

(2) Preparation of Separator Having Coating Layer Containing PolyamicAcid Formed Thereon

The above prepared coating composition was coated on both surfaces ofthe polyethylene substrate film having a thickness of 12 μm in a mannerof dip coating, and then dried at room temperature for 24 hours toprepare a separator.

Example 2

The separator was prepared by the same method as in above Example 1,except that 1,6-hexamethylenediamine was placed in the four-neck flaskinstead of 3,3′-diaminodiphenylsulfone, and the mixing was carried outin a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:1.5:4.1:3.5, in the preparation of the coatingcomposition.

Example 3

The separator was prepared by the same method as in above Example 1,except that 4,4′-oxydianiline was placed in the four-neck flask insteadof 3,3′-diaminodiphenylsulfone, the solid content of the polyamic acidsolution by mass ratio was 10% by weight, and the mixing was carried outin a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:2.8:4.1:2.2, in the preparation of the coatingcomposition.

Example 4

The separator was prepared by the same method as in above Example 1,except that 0.2 mol of 4,4′-oxydianiline and 0.3 mol of3,3′-diaminodiphenylsulfone were placed in the four-neck flask insteadof 0.5 mol of 3,3′-diaminodiphenylsulfone, and the mixing was carriedout in a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:1.5:4.1:3.5, in the preparation of the coatingcomposition.

Example 5

The separator was prepared by the same method as in above Example 1,except that 4,4′-diaminodiphenylsulfone was placed in the four-neckflask instead of 3,3-diaminodiphenylsulfone, in the preparation of thecoating composition.

Example 6

The separator was prepared by the same method as in above Example 1,except that 0.375 mol of pyromellitic dianhydride and 0.125 mol of4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used insteadof 0.5 mol of pyromellitic dianhydride, and the mixing was carried outin a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:0.2:4.1:4.7, in the preparation of the coatingcomposition.

Example 7

The separator was prepared by the same method as in above Example 1,except that polyvinylidene fluoride homopolymer (hereinafter, referredto as ‘PVdF homopolymer’) solution was further included, and the mixingwas carried out in a weight ratio of the prepared polyamic acidsolution:PVdF homopolymer solution:N,N-dimethylacetamide(DMAc):inorganic dispersion:acetone=0.5:0.7:0.7:4.1:4.0, in thepreparation of the coating composition.

The PVdF homopolymer solution was prepared by adding 10% by weight ofPVdF homopolymer (5130, Solvay) to DMF (Daejung Chemical & Metals Co.,Ltd.), and stirring it at 25° C. for 4 hours using a stirrer.

Example 8

The separator was prepared by the same method as in above Example 1,except that polyvinylidene fluoride-hexafluoropropylene copolymer(hereinafter, referred to as ‘PVdF-HFP copolymer’) solution was furtherincluded, and the mixing was carried out in a weight ratio of theprepared polyamic acid solution:PVdF-HFP copolymersolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.5:0.7:1.4:4.1:3.3, in the preparation of thecoating composition.

The PVdF-HFP copolymer solution was prepared by adding 10% by weight ofPVdF-HFP copolymer having a weight average molecular weight of 700,000g/mol (21216, Solvay) to acetone (Daejung Chemical & Metals Co., Ltd.),and stirring it at 25° C. for 4 hours using a stirrer.

Example 9

The separator was prepared by the same method as in above Example 1,except that 0.45 mol of pyromellitic dianhydride and 0.05 mol of4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used insteadof 0.5 mol of pyromellitic dianhydride, and the mixing was carried outin a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:0.3:4.1:4.7, in the preparation of the coatingcomposition.

Example 10

The separator was prepared by the same method as in above Example 1,except that 0.35 mol of pyromellitic dianhydride and 0.15 mol of4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used insteadof 0.5 mol of pyromellitic dianhydride, and the mixing was carried outin a weight ratio of the prepared polyamic acidsolution:N,N-dimethylacetamide (DMAc):inorganicdispersion:acetone=0.9:0.1:4.1:4.7, in the preparation of the coatingcomposition.

Example 11

The separator was prepared by the same method as in above Example 1,except that 0.3 mol of pyromellitic dianhydride and 0.2 mol of4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used insteadof 0.5 mol of pyromellitic dianhydride, and the mixing was carried outin a weight ratio of the prepared polyamic acid solution:inorganicdispersion:acetone=0.9:4.1:4.7, in the preparation of the coatingcomposition.

Example 12

The separator was prepared by the same method as in above Example 1,except that 1,2,3,4-Butanetetracarboxylic dianhydride was used insteadof pyromellitic dianhydride, in the preparation of the coating agent.

Example 13

The separator was prepared by the same method as in above Example 1,except that 1,2,4,5-cyclohexanetetracarboxylic dianhydride was usedinstead of pyromellitic dianhydride, in the preparation of the coatingagent.

Example 14

The separator was prepared by the same method as in above Example 1,except that 4,4′-carbonyldiphthalic dianhydride was used instead ofpyromellitic dianhydride, in the preparation of the coating agent.

Example 15

The separator was prepared by the same method as in above Example 1,except that 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, and1,16-hexadecanediamine were used instead of pyromellitic dianhydride,and 3,3′-diamino diphenyl sulfone, respectively, in the preparation ofthe coating agent.

Example 16

The separator was prepared by the same method as in above Example 1,except that 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, and1,4-cyclohexyldiamine were used instead of pyromellitic dianhydride, and3,3′-diamino diphenyl sulfone, respectively, in the preparation of thecoating agent.

Example 17

The separator was prepared by the same method as in above Example 6,except that 4,4′-diamino diphenyl sulfone was used instead of3,3′-diamino diphenyl sulfone, in the preparation of the coating agent.

Example 18

The separator was prepared by the same method as in above Example 6,except that 1,2,4,5-cyclohexanetetracarboxylic dianhydride was usedinstead of pyromellitic dianhydride, in the preparation of the coatingagent.

Example 19

The separator was prepared by the same method as in above Example 6,except that 4,4′-isopropylidenediphthalic dianhydride was used insteadof 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, in thepreparation of the coating agent.

Example 20

The separator was prepared by the same method as in above Example 1,except that 3-[3-(3-aminophenyl)sulfonylphenyl] sulfonylaniline was usedinstead of, and 0.375 mol of pyromellitic dianhydride and 0.125 mol of4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used insteadof 0.5 mol of pyromellitic dianhydride, in the preparation of thecoating agent.

The solvent ratios used in the above Examples are shown in the followingTable 1.

TABLE 1 Acetone DMAc Example 1 8.15 1.85 Example 2 7.3 2.7 Example 3 6 4Example 4 7.3 2.7 Example 5 8.15 1.85 Example 6 8.94 1.06 Example 9 8.851.15 Example 10 9.04 0.96 Example 11 9.15 0.85 Example 12 8.15 1.85Example 13 8.15 1.85 Example 14 8.15 1.85 Example 15 8.15 1.85 Example16 8.15 1.85 Example 17 8.94 1.06 Example 18 8.94 1.06 Example 19 8.941.06 Example 20 8.15 1.85

Comparative Examples 1 and 2

Preparation of Separator Having a Coating Layer not Containing PolyamicAcid Formed Thereon

Comparative Example 1

A coating agent prepared by mixing in a compositional ratio of thePVdF-HFP copolymer solution of Example 8:the inorganic dispersion ofExample 1:acetone=2:4.8:3.2, and stirring it at 25° C. for 2 hours witha power mixer, was coated on both surfaces of a polyethylene substratefilm having a thickness of 14 μm in a manner of dip coating, and driedto prepare the separator.

Comparative Example 2

The separator was prepared in the same method as in Comparative Example1, except that the PVdF homopolymer solution of Example 7 was usedinstead of the PVdF-HFP copolymer solution.

TABLE 2 Molecular Polyamic acid weight Ex- am- ple 1

40,000~ 60,000 Ex- am- ple 2

60,000~ 80,000 Ex- am- ple 3

90,000~ 110,000 Ex- am- ple 4

80,000~ 100,000 x:y = 4:6 Ex- am- ple 5

60,000~ 80,000 Ex- am- ple 6

60,000~ 80,000 x:y = 7.5:2.5 Ex- am- ple 7

40,000~ 60,000 Ex- am- ple 8

40,000~ 60,000 Ex- am- ple 9

60,000~ 80,000 x:y = 9:1 Ex- am- ple 10

60,000~ 80,000 x:y = 7:3 Ex- am- ple 11

60,000~ 80,000 x:y = 6:4 Ex- am- ple 12

40,000~ 60,000 Ex- am- ple 13

50,000~ 70,000 Ex- am- ple 14

60,000~ 80,000 Ex- am- ple 15

40,000~ 60,000 Ex- am- ple 16

60,000~ 80,000 Ex- am- ple 17

70,000~ 90,000 x:y = 7.5:2.5 Ex- am- ple 18

70,000~ 90,000 x:y = 7.5:2.5 Ex- am- ples 19

60,000~ 80,000 x:y = 7.5:2.5 Ex- am- ple 20

70,000~ 90,000 x:y = 7.5:2.5

EXPERIMENTAL EXAMPLE 1: SOLUBILITY IN LOW BOILING POINT AND HIGH BOILINGPOINT SOLVENTS ACCORDING TO STRUCTURE OF POLYAMIC ACID

In order to evaluate the solubility of the polyamic acids preparedaccording to the above Examples each having different structure in a lowboiling point solvent and a high boiling point solvent, the ratios ofthe low boiling point solvent and the high boiling point solvent atwhich the solvents are maintained in the most clear state when each ofthe polyamic acid is dissolved therein, were measured. Acetone (DaejungChemical & Metals Co., Ltd) was used as the low boiling point solvent,and N,N-dimethylacetamide (DMAc) was used as the high boiling pointsolvent.

As to the low boiling point solvent and the high boiling point solventof each of the polyamic acids, the results of measuring the acetone:DMAcratios at which the most clear state is maintained, are shown in thefollowing Table 3:

TABLE 3 Examples Acetone:DMAc 1 8.75:1.25 2 7.5:2.5 3 6:4 4 6.66:3.34 58.33:1.67 6 9.09:0.91 9  8.88:.1.12 10 9.44:0.56 11 Dissolved in anelectrolyte solution 12 9:1 13  8.88:.1.12 14 9:1 15 9.33:0.67 169.23:0.77 17 9.33:0.67 18 9.41:0.59 19 9.41:0.59 20 9.47:0.53

As shown in the above Table 3, it was confirmed that the polyamic acidsprepared in above Examples 1 to 20 have excellent solubility in acetonewhich is the low boiling point solvent, and thus, are suitable for beingutilized in the coating composition of the separator.

In particular, the polyamic acid having the highest solubility inacetone was those prepared according to Examples 6, 9, 10 and 11, andwhen comparing these polyamic acids, it was confirmed that as the ratioof 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride increases, thesolubility of the polyamic acid in acetone increases.

However, it was observed that the polyamic acid of Example 11 dissolvedalso in an electrolyte solution of a battery, as a result of itsexcessively increased solubility in the low boiling point solvent.

EXPERIMENTAL EXAMPLE 2: MEASUREMENT OF THICKNESS AND COATED WEIGHT OFCOATING LAYER

The following method was carried out in order to measure the thicknessesand the coated weights of the coating layers of the separators preparedin above Examples 1 to 20, and Comparative Examples 1 and 2.

First, the thickness of each coating layer was measured using a SEMcross section image of each coating layer and micro calipers. Then, eachcoating layer was cut into a size of 10 cm (MD)×20 cm (TD) and itsweight was measured with an electronic scale to calculate the coatedweight. The results of measuring the thicknesses and the coated weightsare shown in the following Table 5.

EXPERIMENTAL EXAMPLE 3: MEASUREMENT OF THERMAL SHRINKAGE OF SEPARATOR

The following method was carried out in order to measure the thermalshrinkage of the separators prepared in above Examples 1 to 20, andComparative Examples 1 and 2.

Each separator prepared according to the above Examples and ComparativeExamples was cut into a size of 5 cm (MD)×5 cm (TD), thereby producing atotal of seven samples. Each of the samples was stored in chambers at150° C. and 200° C. for 1 hour, and then shrinkage amounts in MD and TDdirections of each sample were measured to calculate the thermalshrinkage. The results of measuring the thermal shrinkage are shown inthe following Table 5.

EXPERIMENTAL EXAMPLE 4: MEASUREMENT OF AIR PERMEABILITY

The air permeability of the separators prepared in above Examples 1 to20, and Comparative Examples 1 and 2 was measured by determining thetime to pass 100 cc of air through the separator using EG01-55-1MR(Asahi Seiko Co., Ltd.).

EXPERIMENTAL EXAMPLE 5: ELECTROLYTE SOLUTION WETTABILITY OF SEPARATOR

The following method was carried out in order to measure the electrolytesolution wettability of the separators prepared in above Examples 1 to20, and Comparative Examples 1 and 2.

Each separator prepared according to the above Examples and ComparativeExamples was cut into a square of 3 cm (MD)×3 cm (TD), thereby producinga total of seven samples. After each sample was floated on the surfaceof an electrolyte solution in a beaker, time to be completely soaked bythe electrolyte solution was measured.

The time to be soaked by the electrolyte solution is shown in thefollowing Table 5.

EXPERIMENTAL EXAMPLE 6: MEASUREMENT OF DMAC SOLVENT RESIDUAL AMOUNT INCOATING LAYER OF SEPARATOR

In order to measure the organic solvent residual amounts in the coatinglayers of the dried separators prepared in above Examples 1 to 20, andComparative Examples 1 and 2, gas chromatography (HP-6890) was performedunder conditions described in the following Table 4, and the results areshown in Table 5.

TABLE 4 Parameters Conditions Column Front: HP-INNOWax (length 30 M, ID0.53 mm, film thickness 1.00 μm) Back: HP-1 (length 30 M, ID 0.53 mm,film thickness 0.88 μm) Temperature and 40° C. (4 min.) → 20° C./min →250° C. (4 min.) time Flow rate 10 mL/min Injector S/SL Injector Splitratio 5:1 Detector FID Injection volume 1 μl Injector temperature 200°C.

TABLE 5 Thick- Coated Thermal shrinkage of separator nesses of weightsof Solvent Electrolyte Air (%) Coating coating residual solutionpermeability 150° C., 200° C., layer layer amount wettability (Sec/100 1hour 1 hour (μm) (g/m²) (ppm) ( sec) cc) TD MD TD MD Example 1 6.1 7.3120 30 550 0.5 0.6 0.9 1.0 Example 2 6.6 7.5 115 40 650 0.5 0.6 0.9 1.0Example 3 6.9 7.7 310 45 790 0.3 0.6 0.8 0.9 Example 4 6.8 7.6 180 35650 0.4 0.5 0.9 1.0 Example 5 6.4 7.6 125 34 600 0.5 0.5 1.0 1.0 Example6 6.2 7.2 55 31 403 0.5 0.7 1.1 1.3 Example 7 8.1 8.4 80 55 426 1.1 1.12.0 2.1 Example 8 7.8 8.1 85 38 438 1.4 1.3 2.5 3.0 Example 9 6.9 7.8 5532 380 0.5 0.7 0.9 1.0 Example 10 6.3 7.5 56 32 380 0.5 0.6 1.0 1.1Example 11 6.0 7.0 55 30 350 0.6 0.7 1.1 1.2 Example 12 6.2 7.3 115 35360 0.7 0.5 0.9 0.8 Example 13 6.7 7.5 120 37 340 0.6 0.7 0.9 1.0Example 14 6.5 7.4 120 40 330 0.5 0.8 0.8 1.1 Example 15 6.1 7.1 120 41290 0.6 1.0 0.9 1.2 Example 16 6.3 7.3 115 40 295 0.5 0.7 1.0 1.3Example 17 6.0 7.0 60 38 270 1.3 1.4 2.0 2.5 Example 18 6.0 7.0 55 37265 1.4 1.5 2.5 3.5 Example 19 6.0 7.0 60 32 280 0.6 0.8 1.2 1.3 Example20 6.0 7.0 95 40 295 1.0 1.2 1.9 2.1 Comparative 8.8 8.5 — 65 414 21.526.0 — — Example 1 Comparative 9.0 9.0 — >300 455 10.0 12.5 — — Example2

As shown in the above Table 5, it is confirmed that the separators ofExamples 1 to 20 containing polyamic acid in the coating layers haveless thermal shrinkage amounts, and thus, have excellent thermalresistance, as compared with those of Comparative Examples 1 and 2 notcontaining polyamic acid.

Further, when comparing the separators of Examples 6, 9, 10 and 11, asthe ratio of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydrideincreases, the solubility of the polyamic acid in acetone increases (seeTable 1), and thus, it was confirmed that as the solvent is easilyvolatilized, drying performance is improved, and as a result, thesolvent residual amount of the coating layer is small, so that the airpermeability becomes excellent.

Meanwhile, after a lithium secondary battery was manufactured using theseparator according to Example 1, battery capacity change by the use ofthe battery was observed, and as a result, it was confirmed that abattery capacity was hardly changed even after about 350 cycles (FIG. 1).

Accordingly, it is considered that in case of utilizing the separator ofthe present invention in an electrochemical battery, thermal stabilityof the battery may be improved, such that a battery life is extendedover a long term.

EXPERIMENTAL EXAMPLE 7: MEASUREMENT OF SHUTDOWN FUNCTION OF SEPARATOR

In order to measure the shutdown function of each separator according toabove Example 1 and Comparative Example 1, the impedance of theseparators prepared in Example 1 and Comparative Example 1 was measuredat 1 kHz and a heating rate of 10° C./min using 3522-50 LCR HiTester(HIOKI) (FIG. 2 ).

It is shown in FIG. 2 that the shutdown temperatures of Example 1 andComparative Example 1 were similar, and when comparing it with theresults of Experimental Example 3, it was confirmed that the separatorof Example 1 having a coating layer containing polyamic acid formedthereon had improved thermal resistance, while still maintaining theshutdown function excellent, and thus, the high temperature safety ofthe separator may be secured.

EXPERIMENTAL EXAMPLE 8: MEASUREMENT OF TMA (THERMAL MECHANICAL ANALYSIS)

In order to measure TMA of each separator according to above Examplesand Comparative Examples, the temperature at which each separator breakswas measured using TA Instruments (TMA Q400) by pulling each separatorwith a force of 0.005 N at a heating rate of 5° C./min (see ASTM E 831)(FIG. 3 ). The results are shown in the following Table 6.

It is shown in FIG. 3 that the separator of Comparative Example 1 wasobserved to have pore shrinkage in the vicinity of shutdown temperature(about 130° C.), and represented a breaking property at about 150° C. ormore. On the contrary, the separator of Example 1 hardly showedshrinkage up to 191° C., due to the high temperature safety of thecoating layer containing polyamic acid. Further, according to Table 6,the separator including the polyamic acid coating layer showed abreaking temperature of 180° C. or more.

TABLE 6 Breaking temperature (° C.) Example 1 191 Example 2 190 Example3 188 Example 4 187 Example 5 190 Example 6 188 Example 7 176 Example 8179 Example 9 188 Example 10 190 Example 11 189 Example 12 188 Example13 187 Example 14 190 Example 15 190 Example 16 189 Example 17 187Example 18 188 Example 19 191 Example 20 188 Comparative 145 Example 1Comparative 144 Example 2

EXPERIMENTAL EXAMPLE 9: MEASUREMENT OF HIGH-RATE DISCHARGE PROPERTY(C-RATE)

The high-rate discharge property of each separator according to Example1 and Comparative Example 1 was measured (FIG. 4 ). The high-ratedischarge property was measured by observing a capacity change rate bydetermining the capacities of a battery having a capacity of 850 mAh,wherein the capacity was determined after charging for 2 hours and thendischarging for 5 hours (0.2C), after charging for 2 hours and thendischarging for 2 hours (0.5C), after charging for 2 hours and thendischarging for 1 hour (1C), and after charging for 2 hours and thendischarging for 30 minutes (2C), using a charge/discharge tester(TOSCAT-3600, TOYO Systems Co., Ltd.).

It is shown in FIG. 4 that the separator of Example 1 represented a muchexcellent high-rate discharge property as compared with the separator ofComparative Example 1. As the high-rate discharge property is superior,the output property of a battery is improved, and thus, the separator ofExample 1 may significantly contribute to improved safety of a battery,and also development of a higher output battery, due to an excellenthigh temperature property.

The invention claimed is:
 1. A separator, comprising: a polyolefin-basedsubstrate film, and a coating layer positioned on at least one ofsurface of the polyolefin-based substrate film, wherein the coatinglayer is formed from a composition comprising an organic binderincluding a polyamic acid represented by any one of the followingChemical Formulae 5 to 9, a low boiling point solvent having a boilingpoint less than 150° C., and a high boiling point solvent having aboiling point of 150° C. or more:

wherein, in Chemical Formula 5, x is an integer of 15 to 5000,

wherein, in Chemical Formula 6, x is an integer of 15 to 5000,

wherein, in Chemical Formula 7, x is an integer of 15 to 5000,

wherein, in Chemical Formula 8, x is an integer of 15 to 5000, y is aninteger of 15 to 5000, and a ratio x:y is 5:5 to 1:9, and

wherein, in Chemical Formula 9: x is an integer of 15 to 5000, and y isan integer of 15 to 5000, and a ratio of repeating units x to y is 9:1to 7:3.
 2. The separator as claimed in claim 1, wherein the polyamicacid is contained in 1 to 30% by weight, based on a total weight of thecomposition.
 3. The separator as claimed in claim 1, wherein the lowboiling point solvent is selected from acetone, tetrahydrofuran (THF),or combination thereof.
 4. The separator as claimed in claim 1, whereinthe high boiling point solvent is selected from dimethylformamide (DMF),dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAc),dimethylcarbonate (DMC), N-methylpyrrolidone (NMP), or combinationthereof.
 5. The separator as claimed in claim 1, wherein the low boilingpoint solvent and the high boiling point solvent are included in aweight ratio of 9.5:0.5 to 5:5.
 6. The separator as claimed in claim 1,wherein the low boiling point solvent is acetone and the high boilingpoint solvent is N,N-dimethylacetamide.
 7. The separator as claimed inclaim 1, wherein a residual amount of the low boiling point solvent andthe high boiling point solvent in the separator after drying is 500 ppmor less.
 8. The separator as claimed in claim 1, wherein: the polyamicacid is represented by Chemical Formula 9, and one or more aryl sulfonegroups in the repeating unit x or y of the Chemical Formula 9 are ameta-aryl sulfone group.
 9. The separator as claimed in claim 1, whereinthe separator has a breaking temperature of 180° C. or more under acondition of 0.005 N, and a heating rate of 5° C./min.
 10. The separatoras claimed in claim 1, wherein the composition further containsinorganic particles, and wherein the inorganic particles are containedin 10 to 70% by weight, based on a total weight of the composition. 11.The separator as claimed in claim 1, wherein the composition furthercontains an additional binder selected from the group of polyvinylidenefluoride (PVdF) homopolymer, polyvinylidene fluoride-hexafluoropropylenecopolymer (PVdF-HFP), polymethylmethacrylate, polyacrylonitrile,polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate,polyethylene oxide, cellulose acetate, cellulose acetate butyrate,cellulose acetate propionate, cyanoethylpullulan,cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose,pullulan, carboxyl methyl cellulose, and acrylonitrilestyrene-butadienecopolymer, alone or in a mixture thereof.
 12. The separator as claimedin claim 11, wherein the additional binder is selected frompolyvinylidene fluoride homopolymer, polyvinylidenefluoride-hexafluoropropylene copolymer, or combination thereof.
 13. Amethod of preparing a separator comprising a step of applying acomposition containing a polyamic acid represented by any one of thefollowing Chemical Formulae 5 to 9, a low boiling point solvent having aboiling point less than 150° C., and a high boiling point solvent havinga boiling point of 150° C. or more on at least one of surface of apolyolefin-based substrate film, and a step of drying the appliedcomposition to form a coating layer,

wherein, in Chemical Formula 5, x is an integer of 15 to 5000,

wherein, in Chemical Formula 6, x is an integer of 15 to 5000,

wherein, in Chemical Formula 7, x is an integer of 15 to 5000,

wherein, in Chemical Formula 8, x is an integer of 15 to 5000, y is aninteger of 15 to 5000, and a ratio x:y is 5:5 to 1:9, and

wherein, in Chemical Formula 9: x is an integer of 15 to 5000, and y isan integer of 15 to 5000, and a ratio of repeating units x to y is 9:1to 7:3.
 14. The method as claimed in claim 13, wherein the polyamic acidis mixed in an amount of 1 to 30% by weight, based on a total weight ofthe composition, and the low boiling point solvent and the high boilingpoint solvent are mixed in an amount of 70 to 99% by weight, based onthe total weight of the composition, subsequently the mixture is stirredfor 30 minute to 5 hour on 10 to 40° C.
 15. The method as claimed inclaim 13, wherein the low boiling point solvent and the high boilingpoint solvent are mixed in a weight ratio of 9.5:0.5 to 5:5.
 16. Themethod as claimed in claim 13, wherein the polyamic acid, the lowboiling point solvent, and the high boiling point solvent are mixed withan inorganic dispersion including inorganic particles, and wherein theinorganic particles are mixed in an amount of 10 to 40% by weight, basedon a total weight of the composition.
 17. The method as claimed in claim13, wherein the step of drying is performed for 1 minute to 30 minute at90 to 120° C.
 18. A lithium secondary battery, comprising: a cathode, ananode, the separator as claimed in claim 1, and an electrolyte.